Photochromic glasses or phototropic glasses, as such have also been termed, had their genesis in the U.S. in Patent No. 3,208,860. That patent defines a photochromic glass as having the capability of becoming darker, i.e., changing color, when exposed to actinic radiation and then returning to its original color when the actinic radiation is removed. Customarily, photochromic glass is sensitive to ultraviolet radiation and sometimes to the shorter wavelength portion of the visible spectrum. The patent teaches that such reversible optical properties can be imparted to glass by incorporating silver halide crystals into the glassy matrix. Thus, silver halide crystallites, i.e., crystallites containing AgCl, AgBr, and AgI, are developed in situ via heat treatment of the glass. As is explained in the patent, the mechanism of photochromism is grounded in the phenomenon that the silver halide crystallites, which are submicroscopic in size, are darkenable under the action of actinic radiation and thereby reduce the optical transmittance of the glass. However, when the source of actinic radiation is removed, the crystallites fade to their initial state, restoring the optical transmittance to its original level. The sequence of darkening and fading can be repeated indefinitely under ambient conditions without fatigue or loss of photochromic properties.
U.S. Pat. No. 3,208,860 refers generally to silicate-based host glasses, with the preferred compositions ranging within the R.sub.2 O-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -SiO.sub.2 field. The preferred base glass compositions are stated to consist essentially, expressed in weight percent on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, and 4-26% R.sub.2 O, wherein R.sub.2 O is selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of those components constituting at least 85% of the total composition. To achieve photochromic behavior, the patent discloses adding at least one halide in at least the minimum effective amount of 0.2% Cl, 0.1% Br, and 0.08% I and adding silver in at least the minimum indicated proportion of 0.2% where Cl is the effective halide, 0.05% where Br is the effective halide, and 0.03% where I is the effective halide. Where a transparent glass is desired, the silver content will be maintained below 0.7% and the halide content less than 0.6%. Finally, the patent notes the advantage of including small amounts of low temperature reducing agents, such as SnO, FeO, CuO, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3, to improve the photochromic properties.
The most extensive application to date for photochromic glass has been in the fabrication of ophthalmic lenses, both as prescription lenses and as non-prescription sunglasses. One example of that utility can be found in U.S. Pat. No. 3,197,296 which describes a group of refractive index-corrected silicate glasses containing silver halide crystals to impart the desired photochromic character. In the conventional 2 mm thickness, those glasses exhibited desirable photochromic properties and possessed the necessary refractive index to be compatible with lens grinding practices conventional in the production of prescription ophthalmic lenses.
Prescription lenses, marketed under the trademark PHOTOGRAY.RTM. by Corning Glass Works, Corning, N.Y., have comprised the largest portion of commercial sales. That glass has the approximate composition recited below in weight percent:
______________________________________ SiO.sub.2 55.6% B.sub.2 O.sub.3 16.4 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.65 Na.sub.2 O 1.85 k.sub.2 O 0.01 BaO 6.7 CaO 0.2 PbO 5.0 ZrO.sub.2 2.2 Ag 0.16 Cu 0.035 Cl 0.24 Br 0.145 F 0.19 ______________________________________
Because the composition of PHOTOGRAY.RTM. lenses represents compromises made between photochromic characteristics, ophthalmic properties, the capability for being chemically strengthened, as well as melting and forming capability, extensive research has been continuous to provide a glass demonstrating improved photochromic behavior while still retaining the other physical attributes demanded in the production of ophthalmic lenses.
The dynamics of photochromic behavior are quite complex. For example, a photochromic glass will customarily darken to a lower transmittance when the exposure to actinic radiation occurs at lower temperatures. Moreover, where solar radiation constitutes the actinic radiation, the intensity thereof can vary widely depending upon the time of year, the location of the exposure (angle of declination of the sun), cloud cover, snow cover, air mass value, etc. Moreover, whereas the known photochromic glasses do not strictly conform to Bouguer's Law, because of the absorption of the actinic radiation by the photochromic particles in the glass, the transmittance of a darkened photochromic glass specimen is related in part to the thickness thereof. Thus, where other parameters are held constant, a thicker sample of photochromic glass will normally get darker than a specimen of thin dimensions.
In recent years there has been considerable interest in glass-plastic composite articles and, particularly in the ophthalmic industry, for composite glass-plastic lenses. There are a number of plastics having densities substantially less than glass. As a result, in both the prescription and non-prescription sunglass markets, plastics have seen increasing service since the lightness thereof causes less discomfort to the wearer and has permitted the merchandising of lenses of larger area since the weight, when compared to glass, is much less. Nevertheless, plastic lenses have one drawback which has limited their universal acceptance. Thus, the surfaces of plastic lenses do not possess the surface hardness of glass and, hence, are susceptible to being scratched. Therefore, care must be exercised in handling such lenses. Also, but less importantly, plastics do not exhibit the heat resistance and chemical durability of glass. Accordingly, efforts have been undertaken to produce transparent glass-plastic composite lenses wherein the body of the lens would consist of plastic but at least one surface thereof, conventionally the surface of the lens away from the wearer's face, would have a thin skin of glass laminated thereto. Such a glass surface can provide the desired resistance to surface abrasion, heat resistance, and chemical durability lacking in the plastic. And the resulting composite article would be lighter than a lens formed from glass alone.
Numerous attempts have been made to produce photochromic articles from plastic materials and such articles have been produced. Unfortunately, however, the resultant products have not been practical for use as ophthalmic lenses because each has relatively quickly succumbed to fatigue, i.e., the reversible character of darkening and lightening when subjected to and removed from solar radiation was lost after a few such cycles.
Such results led to research to fabricate glass-plastic composite lenses wherein the glass layer(s), either buried within the plastic or present on the surface, would consist of photochromic glass. Problems in forming glass-plastic composite lenses were legion, particularly with respect to delamination, incomplete bonding, and stress-induced birefringence among others. A successful method for producing sound, transparent glass-plastic composite lenses having a glass surface layer was developed, however, in U.S. application Ser. No. 848,442, filed Nov. 14, 1977 by A. A. Spycher. That application discloses a direct casting method for bonding a glass element to a high-shrinkage thermosetting plastic element whereby the bonding between the elements is secure and the residual stress therebetween is very low. The method comprehends four general steps:
(a) a surface portion of the glass member is coated with a themoplastic adhesive, e.g., polyvinyl butyral, having a heat sealing temperature above the minimum curing temperature of the thermosetting plastic;
(b) casting the thermosetting plastic, e.g., an allyl diglycol carbonate, in liquid form against said surface portion of the glass member which had been coated with the thermoplastic adhesive;
(c) curing the thermosetting plastic by heating the plastic, the glass member, and the thermoplastic adhesive to a temperature below the heat sealing temperature of the thermoplastic adhesive but above the minimum curing temperature of the thermosetting plastic; and then
(d) consolidating the cured thermosetting plastic, the thermoplastic adhesive, and the glass member into an integral glass-plastic composite lens by heating to a temperature above the heat sealing temperature of the thermoplastic adhesive.
Where the microsheet is to be buried within the plastic to form an internal lamina, a similar process to that described above can be employed with slight modifications. Thus:
(a) a predetermined portion of the thermosetting plastic is cast in liquid form against a mold surface finished to optical quality;
(b) a glass microsheet member having surface portions coated with a thermoplastic adhesive exhibiting a heat sealing temperature above the minimum curing temperature of the thermosetting plastic is placed in contact with the liquid plastic;
(c) a further predetermined portion of the thermosetting plastic is cast in liquid form against said glass member;
(d) the thermosetting plastic is cured by heating to a temperature below the heat sealing temperature of the thermoplastic adhesive but above the minimum curing temperature of the thermosetting plastic; and therafter
(e) the cured thermosetting plastic, the thermoplastic adhesive, and the glass member are consolidated into an integral glass-plastic composite lens by heating to a temperature above the heat sealing temperature of the adhesive.
Glass-ceramic molds of the type disclosed in U.S. application No. 839,484, filed Oct. 5, 1977 by A. A. Spycher, are especially suitable for this purpose.
The standard thickness of prescription ophthalmic lenses is about 2 mm (.about.0.080"). In a glass-plastic composite lens to be used for opthalmic purposes, the glass portion will have a refractive index matching that of the plastic and will desirably have a thickness dimension no more than about 0.5 mm (.about.0.20") with a lower limit of thickness of about 0.25 mm (.about.0.010"). A thermosetting plastic exhibiting excellent optical properties and which has been utilized extensively in opthalmic applications under the trademark CR-39.RTM. resin is made from diethylene glycol bis(allyl carbonate) resin and marketed by PPG Industries, Inc., Pittsburgh, PA. That plastic is operable in the above-disclosed method for preparing glass-plastic composite lenses.
In the conventional method for fabricating glass ophthalmic lenses, glass blanks of optical quality are pressed from a melt of molten glass and the blanks are then ground and polished to specified prescriptions. Inasmuch as the thickness of the glass microsheet portion of the glass-plastic composite lens is so small as to have little effect upon the lens prescription, the grinding and polishing of glass lens blanks would be wasteful of both time and material. Hence, it would be far less costly to produce optical quality sheet of photochromic glass having the desired thickness from which shapes of desired configurations and dimensions could be cut for subsequent lamination with plastic.
The making of glass sheet is well known to the art. For example, drawing processes are available which form glass sheet directly from a melt where the glass sheet surfaces are not contacted by mold or roller surfaces until after the glass has cooled to the necessary extent to resist surface marking. Such sheet draw processes include the Colburn process, the Fourcault process, and the Pittsburgh Plate or Pennvernon process. Those methods employ rollers to draw the sheet up from the molten glass but can yield glass of near-optical quality and without substantial surface marking in thicknesses down to about 1.5 mm (.about.0.060"). U.S. Pat. Nos. 3,338,696 and 3,682,609 describe downdraw sheet-forming processes which are especially suitable for the production of very thin, lightweight glass sheet wherein careful control can be had in the forming of uniformly thin microsheet of optical quality.
Unfortunately, however, none of the above sheet drawing procedures provides the rapid melt quenching action inherent in conventional glass pressing practices. Consequently, the sheet drawing processes present a problem for producing haze-free, highly-darkenable photochromic glass sheet. Moreover, by their very nature each of the processes involves maintaining substantial volumes of glass at relatively low temperatures to secure acceptable sheet-forming viscosities in the 10.sup.4 -10.sup.6 range. The molten glass will also be in extended contact with the refractory metals or ceramics which provide the means for forming the drawn sheet. In sum, the sheet-forming processes impose severe constraints on glass composition because of glass stability and liquidus problems intrinsicly associated with the handling and processing of molten glass at rather low temperatures and high viscosities.
Specifically, the glass must possess a viscosity at its liquidus temperature of at least 10.sup.4 poises and, preferably, about 10.sup.5 poises. Furthermore, the glass melt must demonstrate long term stability against devitrification and interfacial crystallization in contact with refractory metals and ceramics such as platinum, mullite, sillimanite, zircon, and high density alumina-containing refractories which have been used to contain and/or form molten glass. This stability should be maintained down to temperatures where the glass exhibits a viscosity between about 10.sup.4 -10.sup.6 poises, the range of viscosities at which the glass is customarily formed.
Methods for sheet drawing photochromic glass have been described in U.S. Pat. Nos. 3,449,103 and 4,018,965 and West German Pat. No. 2,125,232. Neither U.S. Pat. No. 4,018,965 nor West German Pat. No. 2,152,232 deals with the formation of microsheet, i.e., glass having a thickness dimension no greater than about 0.5 mm. U.S. Pat. No. 3,449,103 refers to glass sheet of microsheet thickness but employs contact chilling, e.g., through metal rollers, to quench the melt. Such a process cannot yield optical quality glass sheet of uniform thickness and essentially free from surface defects.
As was observed above, although photochromic glasses do not strictly conform to Bouguer's Law, where other parameters are maintained constant, a thicker specimen of glass will commonly get darker than one of thinner cross section. It will be recogized that where microsheet is involved, i.e., the thickness dimension is no greater than about 0.5 mm, the efficiency of darkening exhibited by a particular photochromic glass must be very high to yield an essentially haze-free product displaying a darkened transmittance of less than about 50% at room temperature. Thus, the composition of the glass must be such that a relatively high percentage of crystallinity is developed, but the size of the crystallites is retained at a low value such as to inhibit the occurrence of haze or cloudiness resulting from diffusion of light passing through the glass. The glass composition is also important in achieving a refractive index in the glass which matches or very closely approximates the plastic material of the composite lens. Moreover, the glass composition plays a vital role in the rate at which the darkening of the glass fades away when the source of actinic radiation is removed.